1. Field of the Invention
The present invention relates to an integrated services digital network (ISDN) system according to recommendation of the CCITT (International Telegraph and Telephone consultative communication) and, in particular, to ISDN network termination units for connecting user's terminal equipments and a subscriber line extending to an ISDN digital switch.
2. Description of the Prior Art
In the ISDN system, a digital subscriber line (DSL) extends from an exchanging office to a user's area. The DSL terminates to a network termination (NT) unit. A user's bus line is connected to the NT unit and extended in the user's area. That is, the NT unit comprises a first port connected to the user's bus line and a second port connected to the DSL. A plurality of terminal equipments (TEs) are connected to the user's bus line in parallel with one another. As TEs, there are known a digital telephone set or sets (which will only be called telephone sets hereinafter), a facsimile set or sets, a personal computer set or sets, and the like.
The CCITT recommendations I series prescribes protocol of user-network connection or interface in connection with seven classified layers which are numbered by 1 through 7. For example, layer 1 is directed to physical connecting conditions such as transmission rate, pin number connectors and others, layer 2 being for data link, layer 3 for call control from a call setup to call release completion.
According to the I series, communication information and control signal are transmitted through different channels which are multiplexed in a time division fashion. The communication information channel is called a B channel and the control channel is referred to as a D channel. It is possible to transmit communication information through the D channel.
As a basic one of user-network interface structures, a basic interface is determined which comprises two B channels of a transmission rate of 64 kbps which are separately called B1 and B2 channels and a single D channel of a transmission rate of 16 kbps.
The basic interface is used for transmission of the communication information and the control signal between the TEs and the NT. That is, the B1 and B2 channels signals and the D channel signals are transmitted between the TEs and the NT as a first time division multiplexed (TDM) signal.
A frame of the first TDM signal of the basic interface will later be described with reference to the drawings.
In connection with the layer 2 for data link, a plurality of logical connections or links are made on D channel by use of LAPD (Link Access Procedure on D channel). Various control information in layer 2 and layer 3 can be transmitted through the links. A frame format of the LAPD is in conformity with the HDLC High-level Data Link Control Procedure) and comprises a start flag, an address field including an identifier code, an information field, a stop flag and others. The frame format will later be described in detail with reference to the drawings.
The NT usually comprises a demultiplexer, a DSL interface and a multiplexer. The demultiplexer is coupled to the first port and demultiplexes the first TDM signal incoming through the first port from one of the TEs to extract the D channel signal in the first TDM signal as a first D channel signal. The first D channel signal is delivered to the DSL interface. The DSL interface is coupled to the second port and transmits the first D channel signal as a second D channel signal in a second TDM signal to the second port. Thus, the second D channel signal is sent out to the DSL through the second port. On the other hand, the DSL interface receives a third TDM signal incoming through the second port from the exchanging office and extracts a third D channel signal in the third TDM signal. The third D channel signal is delivered to the multiplexer. The multiplexer is coupled to the first port and multiplexes the third D channel signal as a fourth control signal in a fourth TDM signal. The fourth TDM signal is delivered to the first port and transmitted to TEs through the user's bus line.
The D channel signal is transmitted from one of TEs to the exchanging office line through the NT and the data link is established. Then, communication information can be transmitted on the data link established. In the similar manner, the D channel signal is also transmitted from the exchanging office to one of TEs which is destined by the address field through the NT and therefore, a data link is also established which extends from the exchanging office to the destined TE.
In the known ISDN system, the data links are established between TEs and the exchanging office through the NT. Therefore, communication between two of the TEs on the same user's bus line is performed by use of the DSL through the exchanging office but cannot be performed without use of the DSL. This is disadvantageous in efficient use of the DSL, in operation rate of the exchanging switch in the office and in user's expenses.